Process for making fluorescent screens, including the elimination of copper impurities



consider the manner in which cathode ray tube screens are commonly made. Accordingly, there s now presented, by Way of example, an outline of a process commonly us-ed for settling tube screens. This basic process yields good results, when modified in the light of the teachings of the present invention. As indicated above, the manner in which the basic process is thus modified will become clear hereinafter.

The basic process which I have used for settling tube screens, consists of agitating the silver-activated zinc sulde and the manganese-activated zinc beryllium silicate (or the other phosphor combinations employed) in a solution including sodium sulfate, lithium hydroxide, potassium silicate and Water. The resultant mixture, after being thoroughly agitated, is introduced into a glass blank of a cathode ray tube supported in such a position that the screen is in a horizontal plane, care being tak-en to prevent splashing of the sides of the blank. The particles suspended in the solution are then permitted to settle out on the screen surface. After the settling has been permitted to proceed for a predetermined length of time-dependent upon the particle size selected, the specific gravity of the overall mixture and the like-the remaining liquid is drained off either by the use of a siphon or by the decanting method, both of which will be Well understood by one familiar with the art to which the invention pertains.

The screen formed by the aforesaid deposition process is then baked, in order to cause it to adhere firmly to the inside surface of the glass face of the tube. Following the baking process, the electron gun and other electrical components of the cathode ray tube are sealed into the glass blank. After the assembly is completed, the tube is evacuated and, during the evacuation process, the entire structure is again :baked at a temperature sufficiently high to cause gases which may be occluded in the component parts to be released while the structure is still connected to the vacuum manifold.

I have found that this baking operation results, at times, in a shift of the color response of the screen. Experimentation has demonstrated that a large part 'of this shift in response, particularly when silver-activated phosphor is included, is due to the presence of varying amounts of impurity components that operate as activators. The principal offender among these impurities I found to be copper, which may be present in a variety of different forms. For example, impurities may be present as copper oxide, copper hydroxide, copper carbonate or copper sulfide.

The primary feature of this invention resides in the use of a scavenging agent in the liquid from Which the phosphor is settled, which agent operates in such manner as to insure that copper impurities are held in soluble form. Substantially all such impurities may then be readily removed When the remaining liquid is removed by siphoning, decantation, or by any desired method employed for this purpose.

The presence of copper impurities may result from a number of different causes. I have found that, not infrequently, extremely small but deleterious quantities of copper impurities are present in the water, or in the sodium sulfate or other chemicals used in the preparation of the settling liquid. Traces of copper may also be present in the apparatus used. Similarly, the phosphors themselves, such as the silver-activated zinc sulde and the manganese-activated zinc beryllillmY silicate, which are preferably utilized, often contain, as contamination, a small percentage of copper. Phosphors which were recently used in the preparation of cathode ray screens in accordance with the present invention Were found to contain small percentages of copper.

As indicated above, and again making particular reference to the drawing, the spectral response characteristics frequently show an excessive response in the green-yellow region.

In Figure 1, curve C represents the spectral response characteristic of pure silver-activated zinc sulfide, Wave length being plotted against relative luminosity, in this figure. Due to the presence of the aforementioned traces of copper impurities, the response more commonly achieved with presumably pure silver-activated zinc sulde approximates the response graphically represented by curve D. Curve E is indicative of the response characteristic of a representative screen consisting of manganese-activated zinc beryllium silicate only.

As set forth, supra, and in the preferred aspect of the invention, it is desired to compose a screen of both silveractivated zinc sulfide and manganese-activated zinc beryllium silicate. While these phosphors may be deposited separately, best results are achieved when both phosphore are settled-out of a single suspension. Prior to the present invention such mixtures did not consistently result in the emission of substantially white light. When the two phosphors indicated are mixed, the presence of the copper impurities, which caused the departure from the desired response characteristic (compare curves C and D, Figure l), results in a curve of the type represented at B in Figure 2, rather than the desired characteristic depicted at A, in that ligure.

Prior to the present invention such mixtures .did not consistently result in the emission of substantially White light. Thus, such double phos-i phor screens, when contaminated with copper impurities, tend to fluoresce too strongly in the green-yellow portion of the spectrum.

In particular accordance with the present invention, the copper impurities are maintained in soluble form by the introduction, into the liquid suspension of phosphor, of a scavening agent which holds the copper impurities in soluble form and makes it possible to remove them at the time of removal of the remaining liquid, and prior to the above-described baking.

I have found that components including a cyanide radical should be used as the scavenging agent, and, more particularly, that the use of potassium or sodium cyanide yields good results. The alkali cyanides, as a class, will yield satisfactory results as the scavening agent, and in general, any cyanide may be used which will result in the formation-in the presence of the copper impurities-of a soluble copper-cyanide complex, providing that it yields less copper on ionizing than is required to form insoluble copper sulfide.

The amount of the scavenging substances to be added is dependent upon the quantities of copper impurities present. In practice it has proved that the addition of potassium or sodium cyanide, ranging in amount from about one-tenth percent to about one-half percent of the suspension liquid, by Weight, achieved good results when commercially available phosphors were employed.

The scavenging agent, when added to the settling liquid at room temperature, reacts with the copper components, whether they be in soluble or insoluble form, forming soluble potassium cuproars-euere` cyanide, K1 Cu (CNM, which is the potassium salt of the complex anion Cu (CN)4". This anion shows extremely low ionization into Cu and. CN

The operation of and necessity for the scavenging agent will be better understoo-d when it is borne in mind that the zinc sulfide phosphor, when suspended in Water, undergoes a, hydrolysis and, to a small extent, ionizes into zinc hydroxide and hydrogen sulde. Copper in any of the simple compounds listed above Would be precipitated as copper sulde by the hydrogen sulfide. Copper held in solution as potassium cuprocyanide is not precipitated by hydrogen sulde. This is the case because of the fact that the concentration of copper ions resluting from ionization of the soluble potassium cuprocyanide, is not suicient to exceed the copper ion concentration (expressed in the solubility product of cuprous sulfide) required to cause precipitation of the slightly soluble copper sulfide.

Alkali cyanides dissolve copper oxide, copper hydroxide, copper carbonate, copper sulfide, etc., which substances change to soluble alkali cuprocyanides, rather rapidly. In these solutions, the concentration of the copper ion, as mentioned above, is so low that any hydrogen sulfide present forms insuiiicient cupric suliide to be precipitated under these conditions.

I have found that the purposes of the present invention are best served when the hydrogen ion concentration of the settling solution lies in the range between pH 10.0 and pH 11.5. By keeping the hydrogen ion concentration in this range, and by having potassium or sodium cyanide present in the solution, substantially all of the copper impurities may be eliminated, as the screen settles out.

It is to be understood that the invention in its broader aspect, is not limited to the use of sodium or potassium cyanide. As indicated above other scavenging agents may be used to cause the copper impurities to be dissolved and held in solution as soluble complexes, rather than permitting them to settle With the other material, to the screen surface.

Further, it is to be understood that by the process of the present invention copper impurities (soluble or insoluble) are removed, Whether they be originally introduced with the Water, present in the apparatus, included with the phosphor, or present in the other chemical constituents used. The appended claims are to be understood with this in mind.

I claim:

l. In a process for settling phosphor out of a liquid suspension of said phosphor, which liquid suspension contains traces of copper impurities in normally insoluble form, the improvement which consists in utilizing a soluble alkali cyanide dissolved in the liquid of said suspension and effective to convert the copper impurities into soluble cuprocyanide complexes, and removing the supernatant liquid and the soluble complexes included therein.

2. In a process for settling phosphor out of a liquid suspension of said phosphor, which liquid suspension contains traces of copper impurities in normally insoluble form, the improvement which consists in utilizing sodium cyanide dissolved in the liquid of said suspension and effective to convert the copper impurities into soluble cuprocyanide complexes, and removing the supernatant liquid and the soluble complexes included therein.

3. In a process for settling phosphor out of a liquid suspension ofV said phosphor, which liquid suspension contains traces of copper impurities in normally insoluble form, the improvement which consists in utilizing potassium cyanide dissolved in the liquid of said suspension and effective to convert the copper impurities into soluble cuprocyanide complexes, and removing the supernatant liquid and the soluble complexes included therein.

4. In a process for settling phosphor, including silver-activated zinc sulphide, out of a liquid suspension of said phosphor, which liquid suspension contains traces of copper impurities in normally insoluble form, the improvement which consists in utilizing a soluble alkali cyanide dissolved in the liquid of said suspension and eiTective to convert the copper impurities into soluble cuprocyanide complexes, and removing the supernatant liquid and the soluble complexes included therein.

5. In a process for settling phosphor, including silver-activated zinc sulphide, outl of a liquid suspension of said phosphor, Which liquid suspension contains traces of copper impurities in normally insoluble form, the improvement which consists in utilizing sodium cyanide dissolved in the liquid of said suspension and effective to convert the copper impurities into soluble cuprocyanide complexes, and removing the supernatant liquid and the soluble complexes included therein.

6. In a process for settling phosphor, including silver-activated zinc sulphide, out of a liquid suspension of said phosphor, Which liquid suspension contains traces of copper impurities in normally insoluble form, the improvement which consists in utilizing potassium cyanide dissolved in the liquid of said suspension and effective to convert the copper impurities into soluble cuprocyanide complexes, and removing the supernatant liquid and the soluble complexes included therein.

7. A process in accordance with claim 4, and further characterized in that the hydrogen ion concentration of the liquid suspension is maintained in the range between pH 10.0 and pI-I 11.5.

8. In the art of producing fluorescent screens, the steps which comprise: providing a settling liquid including sodium sulphate, lithium hydroxide, potassium silicate and water; introducing silver-activated zinc sulphide and manganess-activated zinc beryllium silicate into said liquid; agitating the resultant mixture to cause said silver-activated zinc sulphide and said man- 55 ganese-activated zinc beryllium silicate to be suspended in said liquid; including in the liquid a soluble alkali cyanide to convert any copper impurities, which may be present, into soluble cuprocyanide complexes; and removing the 60 supernatant liquid and the soluble complexes included therein.

9. A process in accordance with claim 8, and further characterized in that said soluble alkali cyanide is present in amount ranging from about 65 one-tenth percent to about one-half percent of the suspension liquid, by weight.

l0. In the art of producing fluorescent screens, the steps which comprise: providing an aqueous suspension of phosphor, said suspension contain- 70 ing traces of copper impurities in normally in- .soluble form; and utilizing a soluble alkali cyanide, dissolved in the suspension liquid, to bring the copper impurities into solution as soluble cuprocyanide complexes.

11. In the art of producing Iluorescent s-creens.

7 the steps which comprise: providing an aqueous suspension of silver-activated zinc sulphide, said suspension containing traces of copper impurities in normaliy insoluble form; and utilizing a soluble alkali cyanide, dissolved in the suspension liquid, to bring the copper impurities into solution as soluble cuprocyanicle complexes.

JOHN J. GEOGHEGAN.

REFERENCES CITED The following references are of record in the le of this patent:

8 UNITED STATES PATENTS Number Name Date 2,075,399 Levy Mar. 30, 1937 2,219,929 Kaufmann Oct. 29, 1940 OTHER REFERENCES Transactions of Illum. Eng. Soc., May 1939, article by Marsden et al., vol. 34, No. 5, pp. 509, lo 510, 512 and 513.

Mellors Modern Inorganic Chemistry, 1939, pp. 594 and 595. 

1. IN A PROCESS FOR SETTLING PHOSPHOR OUT OF A LIQUID SUSPENSION OF SAID PHOSPHOR, WHICH LIQUID SUSPENSION CONTAINS TRACES OF COPPER IMPURITIES IN NORMALLY INSOLUBLE FORM, THE IMPROVEMENT WHICH CONSISTS IN UTILIZING A SOLUBLE ALKALI CYANIDE DISSOLVED IN THE LIQUID OF SAID SUSPENSION AND EFFECTIVE TO CONVERT THE COPPER IMPURITIES INTO SOLUBLE CUPROCYANIDE COMPLEXES, AND REMOVING THE SUPERNATANT LIQUID AND THE SOLUBLE COMPLEXES INCLUDED THEREIN.
 10. IN THE ART OF PRODUCING FLUORESCENT SCREENS, THE STEPS WHICH COMPRISE: PROVIDING AN AQUEOUS SUSPENSION OF PHOSPHOR, SAID SUSPENSION CONTAINING TRACES OF COPPER IMPURITIES IN NORMALLY INSOLUBLE FORM; AND UTILIZING A SOLUBLE ALKALI CYANIDE, DISSOLVED IN THE SUSPENSION LIQUID, TO BRING THE COPPER IMPURITIES INTO SOLUTION AS SOLUBLE CUPROCYANIDE COMPLEXES. 